1. Field of the Invention
This invention relates to the formation of polymeric particles (ie particles having an external surface of organic polymer) which give improved stability when dispersed in a liquid.
2. Description of the Related Art
Processes are described in PCT/GB96/03233 for forming particulate compositions comprising particles having a hydrophilic core within a shell comprising a membrane comprising an association product of (a) an interfacial condensation product (IFC) formed by reaction in a non-aqueous liquid between a first IFC reactant having at least two first condensation groups and the second IFC reactant having at least two second condensation groups and (b) an amphipathic polymeric stabiliser which will concentrate at the interface between oil and water and which has recurring hydrophobic groups and recurring reactive hydrophilic groups which associate with the second condensation groups. After formation in the non-aqueous liquid, the particles are dispersed in liquid electrolyte.
It is explained in that application that the association may comprise a condensation reaction and, in particular, condensation may occur when the stabiliser is a copolymer of an ethylenically unsaturated carboxylic anhydride such as maleic anhydride and the second condensation groups are amino groups. It is also explained, however, that the association preferably comprises forming an internal, ring-formed salt between the adjacent carboxylic groups of a stabiliser formed from a monomer such as maleic acid or maleic anhydride with an IFC reactant which is a polyamine.
We have found that the best performance is generally achieved when the formation of the IFC shell does depend on the use of a polycarboxylic stabiliser which is in hydrolysed acid form rather than anhydride form, and this is probably due to the fact that internal salt formation occurs and that covalent reaction between the amine and the carboxylic acid groups does not occur during normal processing.
We have also found that when particles are made in this way, the resultant particles sometimes have less dispersion-stability than is desirable, especially when they are subsequently dispersed in an aqueous electrolyte solution (such as a liquid detergent concentrate).
We have found that, when developing these unpublished processes, it is difficult simultaneously to optimise the shell formation and the stability of the particles in the final liquid dispersion. We believe that this may arise because of there being different requirements for optimum shell formation and for optimum stability, and because of the differences in the continuous phase. We believe that optimum shell formation may often be promoted by some degree of ionic association between the stabiliser and IFC reactant groups, but we believe that materials which are optimum for undergoing this ionic association may give less satisfactory stability in the final dispersion. Conversely, materials which may give optimum stability in the final dispersion appear to give less adequate shell formation.
Our object, arising out of this unpublished work, is to try to obtain a better combination of properties during manufacture and during long term storage in the electrolyte.
As regards information which has been published, it is well known to stabilise a dispersion of particles in a liquid (for instance a non-aqueous liquid) by including in the dispersion an amphipathic polymeric stabiliser, that is to say a stabiliser formed from hydrophobic groups and hydrophilic (generally ionic) groups. When, as is common, the stabiliser is used for stabilising hydrophilic polymer particles in a hydrophobic environment, the hydrophilic units in the polymer are attracted to the polymer particles and the stabiliser thereby becomes attached to the polymer particles. The mechanism in many instances may be ionic but other types of physical adsorption may be utilised.
A difficulty with this type of stabiliser is that its performance properties are significantly influenced by the nature of the liquid in which the particles are dispersed. If that liquid is changed then the stabiliser may become much less effective. For instance the stabiliser may be effective when the particles are dispersed in a non-aqueous liquid but may be much less effective or wholly ineffective when the continuous phase (the first liquid) of the initial dispersion is changed to a different continuous phase (a second liquid). This change may be brought about by a solvent swap techniques as described in WO94/25560 or it may be brought about by dispersing the initial dispersion into the second liquid. Particular problems arise when the second liquid contains electrolyte.
It would therefore be desirable to be able to improve the self-stabilising properties of polymer particles so that they have the potential to have improved stability even though the continuous phase may be changed from a first (usually non-aqueous) liquid to a different, second, liquid (which is usually aqueous electrolyte).
Stabilisers for different dispersions are known from, for instance, GB-A-1,198,052, GB-A-1,231,614, GB-A-1,268,692, GB-A-2,207,681, AU-A-455,165, U.S. Pat. No. 3,580,880, U.S. Pat. No. 3,875,262, EP-A-707,018 and EP-A-719,085.